Unwritten Legends uses an unusual experience system. When you gain experience, it goes into a buffer that slowly drains over time. This is meant to promote roleplay, as when you have experience in your buffer your character will be advancing in level as fast as possible. The system is also meant to slow down character growth while avoiding forcing players to "grind" for experience.
Any action that causes experience gain always is positively affected by how difficult the action is overall, how long it took to take the action, and how difficult the action is for your character. This keeps experience within an appropriate range for your character.
Combat has a few special rules for experience gain. You will never gain experience from hitting something that is more than 10 levels below you, or less than 3/4 of your level, whichever is lower. When you kill a mobile, you and everyone who killed it gains kill experience. This is an extra boost of experience due to how dangerous combat is in comparison to other tasks.
The experience buffer lets you know how much time is left until it is completely drained, and increases the amount of time it can store as you gain levels. Eventually you will be able to store days of playtime in the buffer. Unfortunately, if you die, you will lose half of your experience buffer. If you end up decaying and going to Ul'Mydar, you lose all of it.
You can gain experience that bypasses the experience buffer through a Role Playing Award (RPA). RPAs are awarded for good roleplaying that staff takes notice of. Since RPAs bypass the experience buffer, roleplaying after you've earned a good amount of time in the experience buffer is an even better idea.
In order to have an idea of how full your experience buffer is, Unwritten Legends also provides something called mental states. In ascending order, the mental states are:
You are musing over a few ideas.
You are reflecting upon recent events.
You are turning things over in your mind.
You are slowly pondering a succession of ideas.
You have attained a state of deep reflection.
Your mind bends beneath the weight of ponderous thoughts.
Your mind is working overtime to digest your recent experiences.
You are meditating heavily upon recent events.
Your head feels as though it is about to explode.
Your mind gyrates wildly and your thoughts spin out of control.